Human capsaicin receptor and uses thereof

ABSTRACT

This invention provides novel human capsaicin receptors and the nucleotide sequences encoding these receptors. Also provided are vectors encoding these receptors and mammalian and non-mammalian cells expressing these vectors. Further provided are assays for identifying compounds that modulate capsaicin receptors and diagnostic assays for capsaicin receptor polymorphisms and aberrant capsaicin receptor expression.

This application is a continuation of U.S. patent application Ser. No. 09/667,422 filed Sep. 21, 200, which issued as U.S. Pat. No. 6,482,611 on Nov. 19, 2002, and which claims priority from U.S. Provisional Application 60/115,617, filed Sep. 23, 1999.

BACKGROUND OF THE INVENTION

The sensation of pain can be triggered by any number of physical or chemical stimuli. In mammals, the peripheral terminals of a group of specialized small diameter sensory neurons, termed “nociceptors” mediate this response to a potentially harmful stimulus.

In efforts to discover better analgesics for the treatment of both acute and chronic pain, and to develop treatments for various neuropathic pain states, considerable research has been focused on the molecular mechanism of nociception. The response to heat, low extracellular pH, or capsaicin (the compound responsible for the hotness of hot peppers) is characterized by the persistent activation of nociceptors (Bevan and Gepetti, 1994, and Kress and Reeh, 1996). It has been shown that both heat and capsaicin are capable of activating dorsal root ganglion and trigeminal ganglion neurons via and influx of cations (Oh, et al. 1996, Kirshstein, et al. 1997). Additionally, moderately acidic conditions produce this response (Zeilhofer, et al., 1997) and also potentiate the response of nociceptors to heat and capsaicin (Kress, et al., 1996).

Capsaicin responses in isolated sensory neurons show dose-dependence and are also evoked by structural analogues of capsaicin (Szolcsanyi and Jancso-Gabor, 1975 and 1976). Resiniferatoxin (RTX), a natural product of Euphorbia plants is a particularly potent activator of the capsaicin response (Szallasi and Blumberg, 1989). Capsaicin and resiniferatoxin share a common vanilloid moiety, thus the capsaicin receptor is also termed the vanilloid receptor (VR). The capsaicin response is competitively inhibited by another structural analog, capsazepine (Bevan, et al., 1992) and by the non-selective cation channel blocker ruthenium red (Wood, 1988).

It was initially postulated that the VR is a non-selective cation channel with a preference for calcium. Consequently, a ⁴⁵Ca²⁺-uptake assay using intact rat dorsal root ganglion (DRG) neurons has been used extensively to characterize structure-activity relations for vanilloids. Specific binding of [³H]RTX provided the first unequivocal proof for the existence of a VR and has furnished a new, biochemical tool to study VR pharmacology. Such studies, however, have been limited by the lack of availability of cloned VR species and sub-types, by the low levels of VR produced by the few cell types that naturally express such receptors in vivo, and by the limited expression levels heretofore obtained using transient recombinant expression technologies.

Interest in characterizing VRs led to the cloning of a functional rat capsaicin receptor (VR1), from a rat dorsal root ganglion cDNA library (Caterina, et al., 1997). The cDNA for the rat capsaicin receptor VR1 encodes an 838 amino acid protein (SEQ ID NO:9) with a predicted molecular mass of 95,000 Daltons.

Sequence analysis suggests that the receptor is composed of a 432 amino acid hydrophilic amino terminus that contains a proline-rich region followed by three ankyrin repeat domains, a membrane bound region that includes 6 beta-sheet transmembrane domains as well as an additional membrane-associated region between transmembrane segments 5 and 6, and a 154 amino acid carboxy terminus.

VR1 is activated not only by vanilloids but also by noxious heat and low pH. As predicted, this VR1 is a relatively non-selective cation channel with a preference for calcium. In Xenopus oocytes expressing VR1, vanilloids evoke inward currents, with RTX being approximately 20-fold more potent (EC₅₀=39 nM) than capsaicin (EC₅₀=710 μM). In VR1-transfected mammalian (HEK293) cells, capsaicin induces whole-cell currents with a potency of 110 nM. Taken together, these results suggest that VR1 corresponds to the site in DRG neurons that mediates calcium uptake.

Homology searches comparing the cloned rat capsaicin receptor VR1 to other known ligand gated channels have revealed some related receptors. The most highly homologous protein identified to date is the recently identified rat vanilloid-receptor-like protein 1 (VRL-1) (Caterina, et al. 1999). This protein shares approximately 49% identical amino acid residues and overall is 66% similar in sequence to the rat capsaicin receptor, VR1, and is predicted to have a tertiary structure quite similar to that of the capsaicin receptor. While the VRL-1 protein has been reported to respond to high temperatures by allowing an influx of cations, it is not a capsaicin receptor, as it is insensitive to capsaicin and capsaicin analogues.

Another class of receptors that shows some homology to the capsaicin receptor is the TRP (transient release potential) family of putative store-operated calcium channels. (Caterina, et al., 1997) also known as “trp channels”. Members of this family of receptors mediate the entry of extracellular Ca²⁺ in response to the depletion of intracellular Ca²⁺ stores (Clapham, 1996). The capsaicin receptor, while mediating the entry of Ca²⁺ and other cations in response to heat, low extracellular pH and capsaicin and related compounds, does not act as a store-operated calcium channel.

If vanilloid binding and calcium uptake are always mediated by the same receptor, a logical prediction would be that ligands mediating these two responses should display similar structure-activity relationships. With regard to DRG neurons expressing native VRs this is clearly not the case: structure-activity analysis of different vanilloid derivatives revealed that the various compounds have distinct potencies for receptor binding and for inducing ⁴⁵Ca²⁺-uptake in rat DRG neurons. Although some compounds, such as RTX-amide, bind to VRs and evoke calcium influx with similar potencies, other vanilloids show relative selectivity for one or the other response. RTX represents one extreme. It is approximately 25-fold more potent for binding (using intact rat DRG neurons the K_(d) was reported to be 40 pM) than for inducing calcium uptake (EC₅₀=1.0 nM). Capsaicin represents the opposite extreme. It evokes calcium influx with an EC₅₀ of 270 nM but inhibits [³H]RTX binding with a 10-fold lower affinity of 3 uM. The most straightforward explanation appeared to be that RTX binding and calcium uptake detected two distinct classes of VRs. These putative receptors were referred to as R-type (preferentially labeled by RTX) and C-type (displaying a higher potency for capsaicin) VRs, respectively. This model was further supported by the identification of non-neuronal cell lines that exhibited calcium uptake in response to vanilloid stimulation (implying the presence of C-type VRs) but which lacked detectable RTX-specific binding sites.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A. Specific binding of [³H] resiniferatoxin to HEK 293 cells transfected stably with a cDNA encoding the rat VR1 (HEK293/VR1 cells). [³H] Resiniferatoxin displays saturable specific binding to HEK293/VR1 cells. Binding data are from a single experiment; points are mean values of triplicate determinations; error bars indicate S.E.M. The binding curve was generated by a computer fit of the measured values to the Hill equation. Half-maximal binding occurred at a concentration of 56 pM [³H] resiniferatoxin; the maximal receptor density was 222 fmol per one million cells. Note that the Hill coefficient (1.8) is indicative of positive binding cooperativity. Three additional experiments yielded similar results. Data are presented as the mean (+/−S.E.M), and binding parameters are set forth in Example 6, Results.

FIG. 1B. Specific binding of [³H] resiniferatoxin to HEK 293 cells transfected stably with a cDNA encoding the rat VR1 (HEK293/VR1 cells). Expanded view of the concentration range from 0-200 pM of the specific binding of [³H] resiniferatoxin to HEK293/VR1 cells. As a result of the cooperativity index approaching 2, the specific binding curve is sigmoidal in this concentration range. Measured values are from FIG. 1A. The binding curve is hypothetical and was computer-generated using the binding parameters from FIG. 1A.

FIG. 1C. Specific binding of [³H] resiniferatoxin to HEK 293 cells transfected stably with a cDNA encoding the rat VR1 (HEK293/VR1 cells). Scatchard plots of specific [³H] resiniferatoxin binding to HEX 293/VR1 cells and CHO/VR1 cells. Filled circles indicate data for HEK293/VR1 cells; data for CHO/VR1 cells are indicated by triangles. The Scatchard plots are convex due to the positive cooperativity of the binding. Also note that the B_(max) value is approximately twice as high in CHO/VR1 as in HEK293/VR1 cells.

FIG. 2. Scatchard plot of specific [³H] resiniferatoxin binding to rat dorsal root ganglion membranes. Half-maximal binding occurred at a concentration of 60 pM [³H] resiniferatoxin; the maximal receptor density was 302 fmol per one million cells. A Hill coefficient, or cooperativity index, of 1.5 was observed. Compare with FIG. 1C and note that [³H] resiniferatoxin binds to rat dorsal root ganglion membranes expressing native vanilloid receptors and to mammalian cells (HEK293 and CHO) transfected with the cloned rat vanilloid receptor VR1 with similar binding parameters. Results of a single experiment are shown; a second experiment yielded similar results.

FIG. 3A. Inhibition of specific [³H] resiniferatoxin binding to HEK 293/VR1 cells by the typical vanilloid agonists olvanil and capsaicin, the novel vanilloids isovelleral and scutigeral, and the competitive vanilloid receptor antagonist capsazepine. Points represent values from 3 to 6 independent determinations; error bars indicate S.E.M. See Example 6, Results, for calculated K_(I) values.

FIG. 3B. Inhibition by olvanil, capsaicin, and capsazepine of specific [³H] resiniferatoxin binding to HEK293/VR1 cells and to CHO/VR1 cells. Open markers indicate data for HEK293/VR1 cells; closed markers indicate data for CHO/VR1 cells. HEK293/VR1 data are from FIG. 3A. For CHO/VR1 cells, points represent mean values of 2 determinations; error bars indicate range.

FIG. 4A. Time dependence of capsaicin and RTX-evoked calcium mobilization in CHO/VR1 cells. Time dependence of capsaicin-evoked calcium mobilization in CHO/VR1 cells in response to 3 nM, 30 nM and 3,000 nM capsaicin. In FIG. 4A and FIG. 4 b, traces from a representative side-by-side comparison of capsaicin and resiniferatoxin effects are displayed. Note that these two typical vanilloid agonists display substantially different kinetics from the responses they evoke.

FIG. 4B. Time dependence of resiniferatoxin-evoked calcium mobilization in CHO/VR1 cells in response to 0.01 nM, 1 M and 100 nM resiniferatoxin.

DESCRIPTION OF THE SEQUENCE LISTINGS

-   SEQ ID NO:1. Human capsaicin receptor cDNA. -   SEQ ID NO:2. Human capsaicin receptor cDNA. Differs from SEQ ID NO:1     only in the 5′ and 3′ untranslated regions. -   SEQ ID NO:3. DNA encoding Short form of Human capsaicin receptor. -   SEQ ID NO:4. Protein encoded by both SEQ ID NO:1 and SEQ ID NO:2.

Transmembrane domains span residues thusly: TM1=434-455, TM2=480-495, TM3=510-530, TM4=543-569, TM5=577-596, TM6=656-684.

-   SEQ ID NO:5. Protein encoded by SEQ ID NO:3. Transmembrane domains     span residues thusly: TM1=434-455, TM2=480-495. -   SEQ ID NO:6. Amino acid sequence of FLAG epitope. -   SEQ ID NO:7. Amino acid sequence of His6x epitope. -   SEQ ID NO:8. Rat capsaicin receptor VR1 cDNA sequence. -   SEQ ID NO:9. Rat capsaicin receptor VR1 amino acid sequence. -   SEQ ID NO:10. The DNA sequence of a 500 bp BglII fragment of the rat     VR1 cDNA (used as a probe). -   SEQ ID NO:11. The DNA sequence of an 830 bp BamHI/NcoI fragment of     the rat VR1 cDNA (used as a probe). -   SEQ ID NO:12. Forward primer used to clone rat VR1 cDNA. -   SEQ ID NO:13. Reverse primer used to clone rat VR1 cDNA.

SUMMARY OF THE INVENTION

This invention relates to novel human capsaicin receptor polypeptides and the nucleotide sequences encoding them. Also included in this invention are nucleic acid vectors (e.g., plasmids) comprising the nucleotide sequence encoding these receptors, mammalian and non-mammalian cell lines comprising such vectors and thereby expressing at least one of the receptor polypeptides, and purified membranes obtained from such mammalian and non-mammalian cell lines. In certain preferred embodiments the polypeptides are full-length active human capsaicin receptor proteins that are capable of binding to capsaicin or capsaicin analogues. In other embodiments the receptors are naturally occurring truncated human capsaicin receptor proteins that are expressed at higher levels in bacterial cells than are full-length human capsaicin receptor proteins. Such truncated proteins are useful for bacterial expression e.g., for the production of immunogens for use in preparing anti-capsaicin receptor antibodies.

In another aspect, this invention relates to assays for identifying compounds that modulate capsaicin receptors, such assays requiring recombinantly expressed active human capsaicin receptor proteins that are capable of binding to capsaicin or capsaicin analogues. Cell lines that express the human capsaicin receptor are useful for screening compounds for either agonist, reverse agonist or antagonist activity at capsaicin receptors. Compounds that act as agonists at the human capsaicin receptor are useful as flavoring agents or animal repellents, while those that act as antagonists or reverse agonists may be useful as analgesics or anesthetics.

In a separate aspect this invention relates to diagnostic assays for capsaicin receptor polymorphisms and aberrant capsaicin receptor expression levels. Such assays are useful for identifying individuals that are either particularly susceptible or particularly insusceptible to the types of pain mediated by the capsaicin receptor and thereby for determining which individuals will benefit from and which will prove refractory to treatment with modulators of this receptor.

DETAILED DESCRIPTION OF THE INVENTION Nucleic Acids

The present invention provides isolated nucleic acid molecules encoding human capsaicin receptors. Isolated nucleic acid molecules encoding human capsaicin receptors comprise DNA molecules, such as genomic DNA molecules, cDNA molecules, or RNA molecules. In one embodiment the isolated nucleic acid molecule is the cDNA sequence shown SEQ ID NO:1. In a separate embodiment the isolated nucleic acid molecule is the cDNA sequence shown is SEQ ID NO:2. In another embodiment the isolated nucleic acid molecule is the cDNA sequence shown in SEQ ID NO:3. SEQ ID NO:1 and SEQ ID NO:2 differ in their non-coding regions, while SEQ ID NO:3 encodes a shortened (truncated) form of the human capsaicin receptor, as well as containing a different 5′ non-coding region.

The invention also includes an isolated nucleic acid molecule encoding the amino acid sequence of SEQ ID NO:4 and an isolated nucleic acid molecule encoding the amino acid sequence of SEQ ID NO:5, in certain preferred embodiments the nucleic acid molecules encode polypeptides with amino acid sequences corresponding to the sequences starting with amino acid 2 (Lys) of SEQ ID NO:4 or SEQ ID NO:5. Also included is an isolated nucleic acid molecule encoding a human capsaicin receptor sequence comprising the amino acid sequence consisting essentially of SEQ ID NO:5.

It will be apparent to those skilled in the art that due to the degeneracy of the genetic code numerous variants of the described nucleic acid molecules can be created by substituting 1 or more codons without changing the encoded amino acid sequence of the protein product. Additionally, nucleic acid changes may be made in the non-coding region of the nucleic acid sequences without altering the amino acid sequence of the protein product.

The present invention also encompasses DNA and cDNA sequences that encode amino acid sequences which differ from those of the human capsaicin receptor but which do not produce phenotypic changes. Preferably such changes are conservative amino acid changes. By the term “conservative amino acid change” is meant any change from one amino acid to another amino acid considered to have similar characteristics (see, e.g., Schulz & Schirmer, 1990) as set forth in Table I hereto.

TABLE I Amino Acids Grouped by Characteristics Large Basic Side Acidic Side Aliphatic Aromatic Polar Side Chains Chains Side Chains Side Chains Chains Lysine, Aspartate, Leucine, Phenylalanine, Glycine, Arginine, Glutamate Isoleucine, Tyrosine, Alanine, Histidine Valine Tryptophan Proline, Cysteine, Serine, Methionine Threonine, Asparagine, Glutamine

Also within the scope of the present invention are other changes to DNA and cDNA sequences encoding the amino acid sequences of SEQ ID NO:4 and SEQ ID NO:5 are in-frame additions of nucleic acid sequences encoding useful amino acid sequence tags. Such tags are useful as, e.g., antibody recognition sites and as sites contributing strong binding interaction characteristics (such as glutathione-S-transferase binding, biotin binding, or metal chelation binding) that are useful for facilitating protein purification via, e.g. affinity chromatography. Such amino acid sequences are well known in the art, and include, but are not limited to the His-6x epitope (SEQ ID NO:6), which chelates copper and other metal ions and is specifically bound by the Monoclonal Anti-polyhistidine Clone HIS-1 monoclonal antibody (Sigma, St. Louis No.H1029), and the FLAG epitope (SEQ ID NO:7), which is specifically bound by the FLAG-M2 monoclonal antibody (Sigma, St. Louis No. F3165). Techniques for making such modifications are also well known in the art, and may be readily carried out using routine methods or by using commercially available kits, for example, the Sigma Mammalian FLAG Expression Kits (Sigma, St. Louis, e.g., Nos. FL-MA and FL-MC).

Also included in the invention are DNA and cDNA sequences encoding human capsaicin receptors identical to those of SEQ ID NO:4 or SEQ ID NO:5, except in the regions encoding their transmembrane domains (as set forth below in the SEQUENCE LISTING). The transmembrane domains of the capsaicin receptor are believed to be β strands. Sequences consisting essentially of the amino acids Tyrosine, Tryptophan, Valine, Threonine, Glutamine, Methionine, Leucine, Isoleucine, Phenylalanine and Cysteine are known to have high propensities for forming β strands (Chou and Fasman, 1974). Substitution of nucleotides encoding any of these amino acids for nucleotides encoding other amino acids in the transmembrane regions of the capsaicin receptors of SEQ ID NO:4 and SEQ ID NO:5 will result in functional receptor translation products, and are within the scope of the present invention.

Polypeptides

This invention provides isolated human capsaicin receptor polypeptides having the amino acid sequence of SEQ ID NO:4 or SEQ ID NO:5. In certain preferred embodiments the polypeptides have amino acid sequences corresponding to the sequences starting with amino acid 2 (Lys) of SEQ ID NO:4 or SEQ ID NO:5. The amino acid sequence given by SEQ ID NO:4 is the protein product encoded by both SEQ ID NO:1 and SEQ ID NO:2. The amino acid sequence given by SEQ ID NO:5 is the protein product encoded by SEQ ID NO:3.

This invention also encompasses human capsaicin receptors having amino acid sequences that further differ from those exemplified herein, but which do not exhibit phenotypic changes. Such amino acid sequences are described above in the discussion of nucleotide sequences, and further include human capsaicin receptors having amino acid sequences that differ from those of SEQ ID NO:4 and SEQ ID NO:5 in the transmembrane domains of the protein without eliminating capsaicin receptor binding or signaling functions. The regions of the amino acid sequences for the receptor considered to represent the transmembrane domains of the protein are annotated as TM1, TM2, TM3, TM4, TM5 and TM6 in the sequence listing for SEQ ID NO:4 and SEQ ID NO:5. It is predicted that the transmembrane domains of the capsaicin receptor are β strands The amino acids Tyrosine, Tryptophan, Valine, Threonine, Glutamine, Methionine, Leucine, Isoleucine, Phenylalanine and Cysteine are known to have high propensities for being in β strands (Chou and Fasman, 1974). Amino acid sequences of the human capsaicin receptor having of any of these amino acids substituted for other amino acids in the transmembrane domains are encompassed by this invention.

Vectors Encoding the Human Capsaicin Receptor

The present invention additionally provides nucleic acid vectors comprising a nucleic acid sequence encoding a polypeptide with the amino acid sequence of SEQ ID NO:4 or SEQ ID NO:5 or, in certain preferred embodiments, encoding amino acid sequences corresponding to the sequences starting with amino acid 2 (Lys) of SEQ ID NO:4 or SEQ ID NO:5. Such nucleic acid sequences include any of the above-described nucleic acid sequences. Suitable vectors include, but are not limited to, a plasmid or a viral vectors. In order for the vector to be used for recombinant expression of a capsaicin receptor, the nucleic acid sequence encoding the receptor must be operatively linked to a nucleic acid sequence comprising at least one regulatory element in the appropriate orientation for expression. Such regulatory elements are well known to those of skill in the art. Preferred regulatory elements are heterologous regulatory elements, i.e., regulatory elements that are not naturally found operatively linked to nucleic acid sequences encoding human capsaicin receptor polypeptides. Such elements include those obtained from other mammalian species as well as those from non-mammalian vertebrates, invertebrates, microbes and viruses. Particualrly preferered regulatory elements are inducible elements, i.e., elements that do not always stimulate expression (or only stimulate relatively low levels of expression), but that respond to environmental stimuli by increasing expression levels of the operatively linked coding sequences. A preferred inducible element is the tetracycline repressible element found in the commercially available pTET OFF™ plasmid vector.

Propagation of the vectors of the invention in microbial hosts is facilitated by the presence in the vector of sequences that act as origins of replication for microbial DNA synthesis, but such microbial-specific sequences are considered to reduce the rate of success in generating certain recombinant cells (particularly in generating transgenic animals), and thus such microbial sequences may be beneficially excised (e.g., by restriction enzyme digestion) prior to the introduction of the vector into an animal cell (e.g., a vertebrate zygote).

The vectors of the invention may be transformed, transfected, microinjected, or otherwise introduced into suitable host cells to form host cell-vector systems for the expression of a polypeptide of the invention. In certain preferred embodiments, the polypeptide exhibits human capsaicin receptor binding and/or signaling activity.

This invention encompasses any of the above-described vectors adapted for infection or transformation of a bacterial cell. Bacterial expression systems for the expression of membrane receptor proteins are available (Muench, et al. 1995). Bacterial host vector systems are also useful in that (when the appropriate microbial origin of DNA replication is present in the vector) they allow the production of large quantities of DNA or (when an appropriate microbial RNA polymerase transcription initiation site is present in the appropriate orientation for RNA expression) of RNA encoding the polypeptides of the invention. A particularly preferred vector for the bacterial expression of the polypeptides of SEQ ID NO:5 is the be pRSET vector that is commercially available from Invitrogen (Carlsbad, Calif.). Protein expression using this vector can be conveniently induced by adding the lactose analog isopropylthiogalactoside (IPTG) to the bacterial culture medium.

This invention also encompasses the above-described vector adapted for expression in a eucaryotic cell (preferably an insect cell, an amphibian cell, or a mammalian cell) which vector further comprises heterologous regulatory elements allowing expression in the cell operatively linked to a nucleic acid molecule encoding at least one polypeptide of the invention, so as to permit expression thereof.

In one embodiment, the vector is adapted for expression in an insect cell. Plasmids commonly used to generate such vectors for this purpose include the BacPak8 and BacPak9 baculoviral vector plasmids (Clontech, Palo Alto, Calif.). These plasmid vectors typically include a multiple cloning site for the insertion in the appropriate orientation for expression of a DNA fragment comprising the sequence encoding the polypeptide to be expressed, such as any of the cDNA sequences of SEQ ID NO:1, SEQ ID NO:2, or SEQ ID NO:3, a promoter sequence, a bacterial origin of DNA replication, and markers for antibiotic resistance.

This invention also encompasses the above-described vector adapted for expression in an amphibian cell, preferably a Xenopus laevis oocyte. Plasmids that may be used to generate such vectors for this purpose include the pcDNA3.1 vector (Invitrogen, Carlsbad, Calif.).

In a preferred embodiment, the vector is adapted for expression in a mammalian cell. An example of a plasmid commonly used to generate such vectors for expression of polypeptides (e.g., those of the present invention) in mammalian cells is pBK-CMV (Stratagene, La Jolla, Calif.) in which the regulatory elements include the cytomegalovirus promoter, which is activated by proteins that are ubiquitously expressed in vertebrate cells.

This invention provides plasmid vectors designated PT35, PT36, and PT44, which comprise the regulatory elements necessary for expression of DNA in a mammalian cell operatively linked to the DNA encoding human capsaicin receptor polypeptides of the invention in the appropriate orientation so as to permit expression.

These plasmids, PT35, PT36, and PT44 were deposited on Aug. 27, 1999 with the American Type Culture Collection (ATCC) 10801 University Blvd., Manassas, Va. 20110-2209, U.S.A. under the provisions of the Budapest Treaty for the International Recognition of the Deposit of Microorganisms for the Purposes of Patent Procedure and were accorded Patent Deposit Numbers PTA-576, PTA-577 and PTA-578, respectively. In accordance with 37 CFR 1.808 (a)(2), et seq., all restrictions imposed by the depositor on the availability to the public of the deposited materials will be irrevocably removed upon the granting of a U.S. patent from the present application.

Recombinant Cells Expressing the Human Capsaicin Receptor

In another aspect this invention provides a recombinant cell expressing the human capsaicin receptor polypeptides, said cell having been obtained by adding the above-described vectors to a host cell. Such host cells include cells from cell lines, cells from primary cultures, and ova and oocytes A preferred cell of this type is a Xenopus laevis oocyte recombinantly expressing active human capsaicin receptor proteins of the invention. Other preferred recombinant cells of the invention include other amphibian cells, insect cells, or mammalian cells comprising at least one of the above-described vectors of the invention. The present invention thus includes such cells comprising such vectors comprising at least the coding regions of the nucleic acid sequences of at least one of SEQ ID NO:1, SEQ ID NO:2, or SEQ ID NO:3, operatively linked to regulatory sequences in the appropriate orientation for the expression of the encoded polypeptides of the invention.

The vector may be added to the host cell by any of the means commonly practiced by those skilled in the art, including but not limited to infection, transformation, transfection or microinjection. Resulting recombinant cells (typically the progeny of the vector-containing host cell) will preferably express at least 2-fold greater capsaicin or capsaicin agonist binding activity (measured, e.g., as described for vanilloid binding in Example 6 hereto), or more preferably at least 10-fold greater binding activity, or most preferably at least 20-fold greater binding activity than a control host cell to which vector has not been added.

In one embodiment the recombinant cell is an insect cell comprising the above-described plasmid or vector. One commonly used insect cell protein expression system is that of Spodoptera frugiperda cells infected with a baculovirus vector.

The invention further provides a mammalian cell comprising the above-described vector. Both adherent and non-adherent mammalian cell are appropriate for expression of the human capsaicin receptor polypeptides of the invention. The mammalian cell may be a COS-7 cell, CHO cell, human embryonic kidney 293 cell (HEK 293), a U937 cell or any other suitable mammalian cell, including cells in primary cell cultures. The mammalian cell may also be prepared by generating a transgenic animal (preferably the animal is a rodent or a domestic livestock species such as a pig, cow, got, sheep, rabbit, or chicken). Methods for generating such transgenic animals are well known, and while often of low efficiency and technically demanding, are nonetheless routine in the art.

Artificially high expression levels of ion channels typically have toxic effects on cultured cells. Without wishing to be bound by any particular theory of operation, it is believed that when it occurs, such toxicity results from excess entry of ions (e.g., Ca²⁺) into cells expressing artificially high levels of ion channels. The capsaicin receptors of the invention act as ion channels, and their expression at artificially high levels would thus be expected to be limited by such toxicity.

The present invention provides for recombinant mammalian cells (e.g., cells comprising an expression vector) expressing high levels of capsaicin receptors, or capable of expressing high levels of capsaicin receptors (e.g., upon derepression—for example removal of tetracycline from the growth medium of cells comprising expression vectors of the invention using the regulatory elements of the commercial pTET OFF™ inducible plasmid vector). Such cells express higher numbers of capsaicin binding sites than do naturally occurring cells, and are particularly useful for the preparation of membranes containing capsaicin receptors for use in binding assays for the identification of compounds that interact (preferably ones that specifically interact, e.g., by binding to the ligand-binding site of the receptor) with such receptors. Preferably, the recombinant cells of the invention are stably transfected cells.

Preferred cells express at least 5×10⁴ capsaicin receptor ligand binding sites per cell. Preferably the cells express at least 1.5×10⁵ such binding sites per cell. More preferably the cells express at least 3×10⁵, and even more preferably, 3×10⁶ such binding sites per cell. Most preferably, the cells express at least 10⁷ capsaicin receptor ligand binding sites per cell.

Preferred cells express at least 80 fmol of capsaicin receptor molecules per 10⁶ cells. Preferably the cells express at least 250 fmol of capsaicin receptor molecules per 10⁶ cells. More preferably the cells express at least 470 fmol of capsaicin receptor molecules per 10⁶ cells and even more preferably at least 2 pmol of capsaicin receptor molecules per 10⁶ cells. Most preferably, the cells express at least 16 pmol of capsaicin receptor molecules per 10⁶ cells.

Isolated Membranes of Recombinant Cells

In certain of its aspects the present invention provides preparations comprising isolated membranes of the recombinant cells of the invention. Preferably, the isolated membranes should exhibit capsaicin receptor ligand binding activity that is significantly greater, preferably at least 2-fold greater, more preferably at least 10-fold greater and most preferably at least 20-fold greater than that exhibited by control membranes isolated from a control host cell (e.g., a cell of the same cell line used to prepare the recombinant cell of the invention that does not contain any vector, or contains a control vector that does not encode a capsaicin receptor). Preferred membranes contain, per mg of total membrane protein, at least 415 fmol, preferably at least 1.25 pmol, even more preferably 2.35 pmol, particularly preferably 4.2 pmol, and most preferably at least 25 pmol of capsaicin receptor. Membranes can be isolated by any suitable method, such as any of the membrane preparation methods that are routinely used in the art.

Assays for Identifying Modulators of Capsaicin Receptors

In a final aspect, the invention provides methods for determining whether a compound can specifically bind to a capsaicin receptor and methods for determining whether a compound can modulate a capsaicin receptor as either an agonist or an antagonist. Agonist compounds are useful as analgesics. This counter-intuitive result is believed to follow from prolonged receptor desensitization that can occur following exposure to such compounds. Agonists, antagonists and reverse agonists are all useful as analgesics, as well as for the prevention and treatment of other conditions, such as treatment of urinary incontinence, prevention of urinary bladder hyper-reflexia and treatment of certain neuropathic pain states such as post herpetic neuralgia, diabetic neuropathy, carpal tunnel syndrome and phantom limb pain in amputees.

The invention thus provides assays for identifying compounds useful a) as analgesics, b) for the treatment of urinary incontinence, c) for the prevention of urinary bladder hyper-reflexia and d) for the treatment of neuropathic pain states (e.g., post herpetic neuralgia, diabetic neuropathy, carpal tunnel syndrome or phantom limb pain).

The invention in particular provides an assay for determining if a compound binds specifically to capsaicin receptors. This assay comprises contacting an experimental sample of either recombinant cells of the invention or isolated membrane preparation of such cells with a labeled capsaicin agonist and a test compound. A second control sample of either recombinant cells expressing the human capsaicin receptor or an isolated membrane preparation of such cells is contacted only with labeled capsaicin agonist. The unbound labeled agonist is removed from both samples and the amount of bound label in both the experimental sample and the control sample is determined. The amount of bound label in the experimental sample is compared to the amount bound label in the control sample. If the experimental sample exhibits a 2-fold decrease, or more preferably a 5-fold decrease or most preferably a 10-fold decrease in the amount of bound labeled capsaicin agonist the compound in the experimental sample is identified as binding specifically to capsaicin receptors.

In the above-described binding assay the labeled capsaicin agonist may be any agonist that is known to bind specifically to capsaicin receptors, such as capsaicin or resiniferatoxin and may be labeled by any detectable label. Detectable labels include, but are not limited to, radiolabels, fluorescent labels and colorometric labels. A particularly preferred labeled capsaicin agonist is [³H] resiniferatoxin. Removal of unbound label may be accomplished by filtering or washing the samples but is not limited to these methods.

The invention also provides functional assays for identifying compounds that act as modulators of capsaicin receptors. Such assays can be used to classify compounds as agonists or antagonists of the capsaicin receptor.

This invention provides a method for determining whether a compound is a human capsaicin receptor agonist, which comprises contacting a recombinant cell of the invention with the compound under conditions that permit activation of a functional human capsaicin receptor response, detecting a functional increase in human capsaicin receptor activity, and there by determining whether the compound is a human capsaicin receptor agonist.

In one such embodiment the invention provides an assay for determining if a compound is an agonist of capsaicin receptors where the functional response is a change in the concentration of intracellular Ca²⁺. This assay comprises contacting a sample of recombinant cells expressing the human capsaicin receptor with an indicator of intracellular Ca²⁺ concentration to yield indicator-loaded cells. After a sufficient incubation period excess indicator is removed from the cells to yield washed, indicator-loaded cells. A potential agonist compound is added to a sample of the washed, indicator-loaded cells. This sample is the experimental sample; the control sample is comprised of washed, indicator-loaded cells to which no potential agonist compound had been added. The concentrations of intracellular Ca²⁺ experimental and control samples are measured by quantitating a change in the indicator of intracellular Ca²⁺. The concentration of intracellular Ca²⁺ in the experimental cells that have been contacted with a potential agonist compound is compared to the concentration of intracellular Ca²⁺ in the control cells. If the experimental sample exhibits a 1.5-fold increase, or more preferably a 5-fold increase or most preferably a 10-fold increase (or any significant increase) in the concentration of intracellular Ca²⁺ the compound in the experimental sample is identified as a capsaicin receptor agonist. As used herein and in the claims, a significant change (e.g., increse or decrese) is one that is significant to the p≦0.05 level in any standard parametric test of statistical significance, such as the F-test, or the Student's T-test.

Particularly preferred indicators of intracellular Ca²⁺ concentration are membrane permeable calcium sensitive dyes, e.g., Fluo-3 and Fura-2. These dyes produce a fluorescent signal when bound to Ca²⁺. Removal of excess indicator from the indicator-loaded cells may be accomplished by washing or filtering cells, but is not limited to these methods.

This invention provides a method for determining whether a compound is a human capsaicin receptor antagonist, which comprises contacting a cell of the invention with the compound in the presence of a known capsaicin receptor agonist, such as capsaicin or resiniferatoxin, under conditions that permit the activation of a functional capsaicin receptor response, detecting a decrease in human capsaicin receptor activity, and thereby determining whether the compound is a human capsaicin receptor antagonist.

In one embodiment, the assay to identify compounds that act as antagonists of capsaicin receptors comprises contacting a test sample of recombinant cells expressing the human capsaicin receptor with an indicator of intracellular Ca²⁺ concentration and a test compound (prefereably the cells are pre-loaded with the indicator). A second control sample of recombinant cells expressing the human capsaicin receptor is contacted only with the indicator of intracellular Ca²⁺ concentration. After a sufficient incubation period excess indicator of intracellular Ca²⁺ is removed from the test and control cells to yield washed, indicator-loaded test and control cells. An agonist of the capsaicin receptor is added to the washed, indicator-loaded cells to yield agonist-contacted test cells and agonist-contacted control cells. The concentration of intracellular Ca²⁺ in the agonist-contacted test cells and the agonist-contacted control cells is measured by measuring changes in the properties of the indicator of intracellular Ca²⁺ concentration. The concentration of intracellular Ca²⁺ in the agonist-contacted test cells is compared to that in agonist-contacted control cells. A test compound for which this comparison indicates that the concentration of intracellular Ca²⁺ in the agonist-contacted test cells is significantly less, to the p≦0.05 level, than the concentration of intracellular Ca²⁺ in the agonist-contacted control cells is identified as an antagonist of capsaicin receptors.

As in the assay for agonists of the capsaicin receptor, particularly preferred indicators of intracellular Ca²⁺ concentration are the membrane permeable calcium sensitive dyes, Fluo-3 and Fura-2. These dyes produce a fluorescent signal when bound to Ca²⁺. Removal of excess indicator from the indicator-loaded cells may be accomplished by any suitable method, such as washing or filtering cells.

The invention will now be further described with reference to the following examples.

EXAMPLES Example 1 Isolation of Human Capsaicin Receptor DNA Clones

Poly A+ RNA was isolated from frozen human dorsal root ganglia. A complementary DNA (cDNA) library was constructed using the ZAP EXPRESS cDNA SYNTHESIS KIT and the ZAP EXPRESS cDNA GIGAPAK III GOLD CLONING KIT (Stratagene, La Jolla, Calif.) according to the manufacturer's instructions. This library contained approximately 1.1×10⁶ independent clones. 1×10⁶ plaque forming units (PFU) from this library were plated and transferred to nitrocellulose filters for hybridization screening. Two ³²P labeled probes were used for screening the filters. One probe, SEQ ID NO:10, corresponded to a 500 bp BglII fragment of the published rat Vanilloid Receptor 1 cDNA (VR1; Caterina, et al. 1997) The second probe, SEQ ID NO:11, corresponded to an 830 bp BamHI/NcoI fragment of VR1. Filters were hybridized with either probe overnight at 42° C. and subsequently washed at 55° C. in 0.2× SSC/0.1% SDS. Positive phage were isolated and re-screened to obtain a single (clonal) phage. The pBK-CMV phagemid was excised from the clonal lambda phage by using the EXASSIST Helper phage (Stratagene). The resulting plasmids were transformed into E. coli strain XLOLR (Stratagene). Plasmid DNA was isolated by maxi prep for sequencing. The nucleotide sequences of the EcoRI/XhoI inserts of the phagemid clones were determined with an ABI CYCLE SEQUENCING KIT (Perkin-Elmer Applied Biosystems, Foster City, Calif.).

Example 2 Recombinant Cells

A. Transiently Transfected Cells

Plasmids PT35, PT36 and PT44 were isolated from XLOLR E. coli and separately transfected into HEK 293T cells (Edge Biosystems, Gaithersville, Md.) by adding 2 μg plasmid DNA and 12 μl LIPOFECTAMINE Reagent (Life Technologies cat no. 10964-013, Life Technologies, Gaithersville, Md.) per 60 mm culture plate according to the manufacturer's instructions. Cells were grown for 24 hrs at 37° C. then reseeded onto 96-well plates suitable for use in the FLIPR™ Plate Reader (Molecular Devices, Sunnyvale Calif.). Cells were grown an additional 24-48 hours before assay.

Transfected cells were assayed for changes in detectible Ca²⁺ levels using the Calcium mobilization assay described below in Example 5. The results of these experiments are set forth in Table II.

B. Stably Transfected Calls

Cells stably expressing the human capsaicin receptor are selected using the Neomycin resistance marker present in the PT35, PT36 and PT44 plasmids. HEK 293T cells transiently transfected with the PT35, PT36 or PT44 plasmids as described above are grown in media containing the antibiotic G418 for two weeks to isolate cell lines stably expressing the recombinantly expressed human capsaicin receptor.

C. Inducible Stably Transfected Cells

The cDNA encoding SEQ ID NO:4 is subcloned into the pTRE vector (Clontech, Palo Alto, Calif.) for recombinant expression in mammalian cells. Plasmids are transfected with LIPOFECTAMINE™ into Chinese Hamster Ovary (CHO) cells containing the pTET OFF™ Regulator plasmid (Clontech, Palo Alto, Calif.). In these cells, expression of the pTRE plasmid is repressed in the presence of tetracycline but is induced upon removal of the antibiotic. Stable clones are isolated in culture medium containing puromycin (10 μg/ml) and maintained in medium supplemented with tetracycline (1 μg/ml). Cells are grown without antibiotic for 48-72 hours prior to assay to facilitate maximal expression of the human capsaicin receptor.

Example 3 Purified Membranes of Cells Expressing the Human Capsaicin Receptor

The human capsaicin receptor-transfected HEK 293T cells of Example 2 were seeded into 96 well plates and grown to 70-90% confluency. Cells were harvested by centrifugation at 3500× g and washed once by resuspension in ice cold PBS containing protease inhibitors. Cells were lysed by POLYTRON (speed 5, 30 seconds) and centrifuged at 536× g for 10 minutes in order to remove DNA, cellular organelles, and unlysed cells. The supernatant, containing isolated membranes, was decanted to a clean centrifuge tube and centrifuged for 20 minutes at 40,000× g. The resulting pellet was washed twice in ice cold PBS and centrifuged again for 20 minutes at 40,000× g. The supernatant of this step was discarded. The protein concentration of the resulting membrane pellet was measured using the Bio-Rad (Bradford) protein assay. By this measure, a 1 liter culture of cells typically yielded 50-75 mg of total membrane protein.

Example 4 Radioligand Binding Assay for Modulators of Capsaicin Receptors

Binding studies with [³H] resiniferatoxin (RTX) (NEN, Boston) were carried out according to the protocol of Szallasi, et. al (1992) in which non-specific RTX Binding is reduced by adding bovine α₁-glycoprotein (100 μg per tube) after the binding reaction has been terminated. Binding assay mixtures were set up on ice and contained [³H] RTX, non-radioactive ligands, 0.25 mg/ml bovine serum albumin (Cohn fraction V), and 5×10⁴-1×10⁵ human capsaicin receptor-transfected HEK-293T cells. The final volume was adjusted to 500 μl (competition binding assays) or 1.000 μl (saturation binding assays) with the buffer described above. Non-specific binding was defined as that occurring in the presence of 1 μM non-radioactive RTX. For saturation binding, [³H] RTX was added in the concentration range of 7-1,000 pM, using 1 to 2 dilutions. Competition binding assays were performed in the presence of 60 pM [³H] RTX and various concentrations of competing ligands. The binding reaction was initiated by transferring the assay mixtures into a 37° C. water bath and was terminated following a 60 minute incubation period by cooling the tubes on ice. Membrane-bound RTX was separated from free RTX as well as the α₁-glycoprotein-bound RTX by pelleting the membranes in a Beckman 12 benchtop centrifuge for 15 minutes at 14,000× g. The radioactivity of membrane-bound RTX was determined by scintillation counting. Equilibrium binding parameters were determined by fitting the Hill equation to the measured values (Szallasi, et al., 1993) with the aid of the computer program FitP™ (Biosoft, Ferguson, Mo.).

Example 5 Calcium Mobilization Assays

A. Response to Capsaicin or Resiniferatoxin

Human capsaicin receptor transfected HEK 293T cells were seeded into 96 well plates and grown to 70-90% confluency. The cells were then washed once with Krebs Ringer solution. Fluo-3 (Molecular Probes, Eugene, Oreg.) calcium sensitive dye (10 ug/mL) was added and incubated with the cells at room temperature for 1 to 2 hours. The 96 well plates were then washed to remove excess dye. Fluorescence response was monitored upon the addition of either 300 nM capsaicin or 30 nM resiniferatoxin by a FLIPR™ plate reader (Molecular Devices, Sunnyvale Calif.) by excitation an 480 nM and emission at 530 nm. Cells transfected with plasmids PT35 and PT44, encoding the full length human capsaicin receptor (SEQ ID NO:1 and SEQ ID NO:2), typically exhibited signals of 5,000-8,000 Arbitrary Fluorescent Light Units in response to agonist.

TABLE II Cell Type Ave Max Response (RFU) Wild-Type  843 PT35 7257 PT36  805 PT44 5529 Measurement of intracellular calcium levels in the presence of capsaicin. HEK 293T cells transiently transfected with the indicated plasmids were exposed to 300 nM capsaicin. Intracellular calcium levels were quantitated on the FLIPR plate reader. Data are shown in Relative Fluorescent Units (RFU). B. Assays for the Identification of Receptor Agonists and Antagonists

The calcium mobilization assay described above may be adapted for identifying test compounds as having agonist or antagonist activity at the human capsaicin receptor.

In order to identify agonist compounds, recombinant cells of the invention are washed and incubated with Fluo-3 dye as described above. A subset of the incubated cells are then exposed to a 1 μm concentration of at least one candidate agonist compound and the fluorescence response is monitored using a FLIPR™ plate reader (Molecular Devices, Sunnyvale, Calif.). Agonist compounds elicit a fluorescence response at least 2-fold that of recombinant cells exposed only to Fluo-3 dye. Preferred agonists elicit a fluorescence response at least 10 fold, and more preferred agonists elicit a fluorescence response at least 20-fold that of recombinant cells exposed only to Fluo-3 dye.

In order to identify antagonist compounds, recombinant cells of the invention are washed an incubated with Fluo-3 dye as described above. One hour prior to measuring the fluorescence signal, a subset of the cells is incubated with a 1 μm concentration of at least one candidate antagonist compound. The fluorescence response upon the subsequent addition of either 300 nM capsaicin or 10 nM resiniferatoxin is monitored using a FLIP™ plate reader (Molecular Devices). Agonist compounds elicit at least a 2-fold decrease in the fluorescence response relative to that measured in the presence of capsaicin or RTX alone. Preferred antagonist compounds elicit at least a 10 fold, and more preferred antagonists at least a 20-fold decrease in the fluorescence response relative to that measured in the presence of capsaicin or RTX alone.

Example 6 Characterization of Capsaicin Receptors Expressed at High Levels in Cultured Cells

[³H] Resiniferatoxin (RTX) binding and calcium uptake by rat dorsal root ganglion (DRG) neurons show distinct structure-activity relations, suggestive of independent vanilloid receptor (VR) subtypes. To evaluate the hypothesis that binding and calcium uptake detect two distinct classes of VRs in DRG neurons, characterization of RTX binding to the rat capsaicin receptor, VR1, expressed in HEK293 and CHO cells and comparison of the structure-activity relations with those for calcium mobilization was tested. In these binding experiments both typical (capsaicin and olvanil) and novel (isovelleral and scutigeral) vanilloids were included, as well as the competitive VR antagonist capsazepine. Vanilloid binding to HEK293/VR1 cells was compared to that measured in rat DRG neurons expressing native VRs. Calcium mobilization in HEK293/VR1 or CHO/VR1 cells was determined in response to RTX, olvanil, and capsaicin, using a fluorescent method. In addition, capsaicin-induced calcium mobilization in the VR1-transfected cells was measured in the presence of capsazepine or the so-called functional VR antagonist, ruthenium red. Agonist and antagonist potencies determined in the calcium mobilization assays using HEK293/VR1 or CHO/VR1 cells were compared to values measured previously in this laboratory for vanilloid-induced ⁴⁵Ca²⁺-uptake by intact rat DRG neurons.

HEK293/VR1 cells and CHO/VR1 cells bound [³H] RTX with affinities of 84 pM and 103 pM, respectively, with a positive cooperativity (Hill numbers were 2.1 and 1.8). These binding parameters are similar to those determined using rat DRG membranes expressing native rat VRs (a K_(d) of 70 pM and a Hill number of 1.7). The typical vanilloid agonists olvanil and capsaicin inhibited [³H]RTX binding to HEK293/VR1 cells with K_(i) values of 0.4 μM and 4.0 μM, respectively. The corresponding values in DRG membranes were 0.3 μM and 2.5 μM. HEK293/VR1 cells and DRG membranes also recognized the novel vanilloids isovelleral and scutigeral with similar affinities (18 and 20 uM in HEK293/VR1 cells; 24 and 21 μM in DRGs). The competitive vanilloid receptor antagonist capsazepine inhibited [³H]RTX binding to HEK293/VR1 cells with a K_(i) value of 6.2 μM, and to DRG membranes with an affinity of 8.6 μM. RTX and capsaicin induced calcium mobilization in HEK293/VR1 cells with EC₅₀ values of 4.1 nM and 82 nM, respectively. Thus, the relative potencies of RTX (more potent for binding) and capsaicin (more potent for calcium mobilization) are similar in DRG neurons and cells transfected with VR1. We conclude that VR1 may account for both the ligand binding and calcium uptake observed in rat DRG neurons.

Experimental Procedures

Materials. [³H] Resiniferatoxin (RTX; 37 Ci/mmol) was synthesized by the Chemical Synthesis and Analysis Laboratory, NCI-FCRDC, Frederick, Md. Nonradioactive RTX was purchased from Alexis Corp. (San Diego, Calif.) and capsazepine was from RBI (Natick, Mass.). Olvanil was a generous gift from Procter and Gamble Corp (Cincinnati, Ohio). Isovelleral and scutigeral were donated by Olov Sterner (Lund Univ., Sweden). All the other chemicals used were purchased from Sigma (St. Louis, Mo.) unless indicated otherwise.

Molecular Biology. A cDNA encoding the rat vanilloid receptor VR1 was cloned from rat dorsal root ganglion (DRG) total RNA by reverse transcription-polymerase chain reaction using primers based on the published nucleotide sequence (Caterina et al., 1997). The forward primer is set forth below as SEQ ID NO:12 and the reverse primer is set forth below as SEQ ID NO:13. A 2.7 kb cDNA was isolated and the nucleotide sequence was verified to be identical to the published sequence, SEQ ID NO:8. This cDNA was subcloned into pcDNA3.1 (Invitrogen, Carlsbad, Calif.) and pTRE (Clontech, Palo Alto, Calif.) for recombinant expression in mammalian cells of the encoded VR1 polypeptide, SEQ ID NO:9.

Cell Culture. The pcDNA3.1 VR1 plasmid was transfected into human embryonic kidney (HEK293) cells using standard methods. These transfected cells were selected for two weeks in media containing G418 (400 μg/ml) and then maintained as a pool of stably transfected cells. The pTRE VR1 plasmid was transfected into Chinese Hamster Ovary (CHO) cells containing the pTet Off Regulator plasmid (Clontech). In these cells, expression of the pTRE plasmid is repressed in the presence of tetracycline but is induced upon removal of the antibiotic. Stable clones were isolated in culture medium containing puromvcin 10 (μg/ml) and maintained in medium supplemented with tetracycline (1 μg/ml) Cells utilized for assays were grown in culture medium without antibiotic for 48-72 hours prior to use. For radioligand binding experiments, cells were seeded in T175 cell culture flasks in media without antibiotics and grown to approximately 90% confluency. The flasks were then washed with PBS and harvested in PBS containing 5 mM EDTA. The cells were pelleted by gentle centrifugation and stored at −80° C. until assayed. For calcium mobilization assays, cells were seeded into 96-well plates and grown to 70-90% confluency.

Membrane Preparations. Female Sprague-Dawley rats weighing 200-250 g were euthanized under CO₂ anesthesia. The spinal columns were opened and DRGs were collected from all levels into ice-cold physiological saline. DRGs were disrupted with the aid of a tissue homogenizer in an ice-cold buffer (pH 7.4) containing (in mM) KCl 5, NaCl 5.8, CaCl₂ 0.75, MgCl₂ 2, sucrose 320, and HEPES 10. Tissue homogenates were first centrifuged for 10 min at 1000× g (4° C.) to remove the nuclear fraction and debris and then the supernatant from the first centrifugation was further centrifuged for 30 min at 35,000× g (4° C.) to obtain a partially purified membrane fraction. Membranes resuspended in the homogenization buffer were stored at −80° C. until assayed.

Radioligand Binding. Binding studies with [³H] RTX were carried out according to a published protocol (Szallasi et al., 1992) in which non-specific RTX binding is reduced by adding bovine alpha₁ acid glycoprotein (100 μg per tube) after the binding reaction has been terminated. Binding assay mixtures were set up on ice and contained [³H] RTX, non-radioactive ligands, 0.25 mg/ml bovine serum albumin (Cohn fraction V), and either 5×10⁴−1×10⁻⁵ VR1-transfected cells or isolated DRG membranes corresponding to 40 μg of DRG membrane protein. The final volume was adjusted to 500 μl (competition binding assays) or 1,000 μl (saturation binding assays) with the ice-cold HEPES (pH 7.4) buffer solution described above. Non-specific binding was defined as that occurring in the presence of 1 μM non-radioactive RTX. For saturation binding, [³H] RTX was added in the concentration range of 7-1,000 pM, using 1 to 2 dilutions. Competition binding assays were performed in the presence of 30 pM (for DRG membranes) or 60 pM (for VR1-transfected cells) [³H] RTX and various concentrations of competing ligands. The binding reactions were initiated by transferring the assay mixtures into a 37° C. water bath and were terminated following a 60 min incubation period by cooling the tubes on ice. Membrane-bound RTX was separated from free, as well as any alpha₁-acid glycoprotein-bound RTX, by pelleting the membranes in a Beckman 12 benchtop centrifuge (15 min, maximal velocity) and the radioactivity determined by scintillation counting. Equilibrium binding parameters were determined by fitting the allosteric Hill equation to the measured values with the aid of the computer program FitP™ (Biosoft, Ferguson, Mo.) as described previously (Szallasi et al., 1993).

Calcium Mobilization Assays. VR1-transfected cells were seeded into 96-well plates and grown to 70-90% confluency. The cells were then washed once with Krebs-Ringer HEPES buffer (25 mM HEPES, 5 mM KCl, 0.96 mM NaH₂PO₄, 1 mM MgSO₄, 2 mM CaCl₂, 5 mM glucose, 1 mM probenecid, pH 7.4) and resuspended and incubated for 1-2 hours in the above buffer supplemented with FLUO3-AM (2.5-10 μg/ml; Teflabs, Austin, Tex. or Molecular Probes, Eugene, Oreg.) at 37° C. in an environment containing 5% CO₂. In some experiments (as indicated below in the RESULTS), the Krebs-Ringer HEPES buffer was also supplemented with 1 mg/ml bovine serum albumin (Cohn fraction V). The wells were then washed twice with Krebs Ringer HEPES buffer. Agonist (olvanil, capsaicin, or RTX)-induced calcium mobilization was monitored using either FLUOROSKAN ASCENT (Labsystems, Franklin, Mass.) or FLIPR (Molecular Devices, Sunnyvale, Calif.) instruments. Similarly, varying concentrations of the antagonists ruthenium red or capsazepine were added to cells concurrently with agonist (25-50 nM capsaicin). Fluorescence data were collected to 60-180 seconds and the maximum fluorescence signal was determined. For the capsaicin- and olvanil-induced calcium responses, data obtained between 30 and 60 seconds after agonist application were used to generate the EC₅₀ values. Kaleidagraph software (Synergy Software, Reading, Pa.) was utilized to fit the data to the equation: y=a*(1/(1+(b/x)^(c))) to determine the EC₅₀ for the response. In this equation, y is the maximum fluorescence signal, x is the concentration of the agonist or antagonist, a is the E_(max), b corresponds to the EC₅₀ or IC₅₀ value, and finaly, c is the Hill coefficient. Results Rat Capsaicin Receptor VR1-Transfected Mammalian Cells (HEK293 and CHO) and Rat DRG Membranes Expressing Native Rat Vanilloid Receptors Bind [³H] RTX with Similar Parameters.

The association of [³H] RTX (60 pM) to VR1 expressed on HEK293 cells was rapid: within 10 min the specific binding attained approximately 90% of its peak value and it then remained on a plateau between 20 min and 60 min of incubation (a single experiment; data not shown). If dissociation was initiated following a 60 min association, it could be fitted to a 1st order decay curve, yielding a dissociation constant of 0.12+/−0.02 min-1 (two determinations; data not shown). Based on these preliminary experiments, an incubation period of 60 min was selected for the equilibrium binding studies.

[³H] RTX (7-1,000 pM) displayed saturable binding to HEK293/VR1 cells (FIG. 1A). The half-maximal binding occurred at 84+/−11 pM (mean+/−S.E.M.; 4 determinations); at the K_(d), non-specific binding represented approximately 20% of the total binding (not shown). The saturation binding curve was sigmoidal, indicating positive cooperativity (FIG. 1B). A fit to the allosteric Hill equation yielded a cooperativity index of 2.1+/−0.2 (mean+/−S.E.M.; 4 determinations). This binding behavior results in a convex Scatchard plot (FIG. 1C). The B_(max) value was 250+/−24 fmol/10⁶ cells (mean+/−S.E.M.; 4 determinations), corresponding to a receptor density of 1.5×10⁵ binding sites per cell. CHO/VR1 cells bound RTX with similar affinity (a K_(d) of 103+/−13 pM; mean+/−range; 2 determinations) and cooperativity values (a Hill number of 1.9+/−0.1; mean+/−range; 2 experiments). The maximal receptor density was, however, approximately two-fold higher than in the HEK293/VR1 cells (470+/−30 fmol/10⁶ cells; mean+/−range; 2 determinations) (FIG. 1C). The VR1-transfected cells lines bound RTX with parameters similar not only to each other but also to rat DRG membranes expressing native vanilloid receptors (FIG. 2). DRG membranes bound [³H] RTX with a K_(d) of 70+/−10 pM and a B_(max) of 290+/−10 fmol/mg protein (mean+/−range; 2 determinations); the cooperativity index was 1.9+/−2 (mean+/−range; 2 determinations).

Although the CHO/VR1 cells provided a higher level of specific binding, HEK293/VR1 cells were chosen for detailed further analysis for a better comparison with the literature (Caterina et al., 1997; Tominaga et al., 1998).

Vanilloid Agonists and the Antagonist Capsazepine Inhibit [³H] RTX Binding by VR1-Transfected Mammalian Cells and DRG Membranes, Respectively, with Similar Affinities.

For the pharmacological characterization of the RTX-recognition site on rat VR1 expressed in HEK293 cells, four agonists (olvanil, capsaicin, isovelleral, and scutigeral) and an antagonist (capsazepine) were selected (FIG. 3). K_(i) values of the agonists were the following: olvanil, 0.4+/−0.1 μM (n=4); capsaicin, 4.0+/−0.8 μM (n=6); isovelleral, 20+/−4 μM (n=3); and scutigeral, 18+/−3 μM (n=3); all values are mean+/−S.E.M. The competitive antagonist capsazepine inhibited [³H] RTX binding with a K_(i) of 6.2+/−0.7 μM (mean+/−S.E.M.; 5 experiments). These K_(i) values are similar to those determined using rat DRG membranes: olvanil, 0.3+/−0.1 μM; capsaicin, 2.5+/−1.1 μM; isovelleral, 24+/−4 μM; scutigeral, 21+/−3 μM; and capsazepine, 8.6+/−3.5 μM (mean+/−S.E.M.; 3 experiments; Table III) The binding affinities of olvanil, capsaicin and capsazepine were also determined using CHO/VR1 cells: K_(i) values were 0.26+/−0.5 μM, 1.3+/−0.4 μM, and 6.6+/−1.4 μM, respectively (mean+/−range; 2 determinations; TABLE III).

TABLE III Rat dorsal root ganglion neurons CHO/VR1 cells Binding ⁴⁵Ca²⁺ - Binding Ca²⁺ - affinity uptake affinity mobilization Ligand (nM) (EC₅₀, nM)* (nM) (EC₅₀, nM) Resinifera- 0.07  1 0.13  1.4 toxin capsaicin 2,500 340 1,700  38 olvanil 300 170** 260 216 capsazepine*** 8,600 271 5,100 140 (1,100) Ruthenium no effect 790 no effect 210 red Values are means of at least 4 independent determinations. *From Acs, G. et al. (1996) Mol. Brain Res. 35: 173-182. **From Walpole, C. S. J. and Wrigglesworth, R. (1993) in Capsaicin in the Study of Pain ***For capsazepine, K_(i) values are given; the values in parenthesis (1,100 nM) were obtained in the absence of bovine serum albumin, 1 mg/ml.

Characterization of rat capsaicin receptor VR1-transfected mammalian cells in the calcium mobilization assay using various vanilloid compounds. Capsaicin induced calcium mobilization in HEK293/VR1 cells and CHO/VR1 cells with EC₅₀ values of 82+/−17 nM (mean+/−S.E.M.; n=4) and 38+/−16 nM (mean+/−S.E.M.; n=5), respectively. RTX was more than an order of a magnitude more potent in both cell lines; EC₅₀ values were 4.1+/−1.3 nM in HEK293/VR1 cells (mean+/−S.E.M.; n=5), and 1.4+/−0.8 nM in CHO/VR1 cells (mean+/−S.E.M.; n=4). Capsaicin and RTX differed not only in potency in the calcium mobilization assay, but also in the kinetics of the response as shown by FIG. 4. Using 30 nM capsaicin, a concentration close to the EC₅₀ in CHO/VR1 cells, led to rapid calcium mobilization responses: the maximal fluorescence change occurred within 30 sec. (30 μM capsaicin) to 50 sec. (3 nM capsaicin). By contrast, RTX-evoked calcium mobilization became detectable only after an initial delay (compare FIG. 4A and FIG. 4B). The calcium mobilization response to capsaicin achieved its peak value within 1 min. and then started to decline, suggestive of the development of tachyphylaxis or due to some other aspect of channel gating (FIG. 4A). By contrast, unless high RTX concentrations were used (such as 100 nM, a value almost 100-fold higher than the EC₅₀), resiniferatoxin application resulted in slowly developing but persistent calcium currents. Calcium mobilization in response to 1 nM RTX increased steadily over a 3 min. period after challenge, approaching the maximal response evoked by 100 nM RTX (FIG. 4B). This difference between the kinetics of capsaicin-induced and RTX-induced responses, however, disappeared when high, supramaximal doses were used (30 uM capsaicin or 100 nM RTX; compare FIG. 4A and FIG. 4B). Olvanil evoked the calcium response in CHO/VR1 cell with a potency of 22+/−6 nM (mean+/−S.E.M.; n=7). The time-course of the olvanil-induced calcium mobilization response was similar to that triggered by capsaicin (not shown). When 25 nM capsaicin was administered to evoke calcium mobilization, capsazepine inhibited this response with an IC₅₀ value of 2.4+/−0.5 uM (mean+/−S.E.M.; n=6). This value was, however, shifted by almost an order of magnitude in the presence of 1 mg/ml bovine serum albumin to yield an IC₅₀ value of 0.33+/−0.03 uM (mean+/−S.E.M.; n=5). For the other vanilloids tested in this study, the presence or absence of bovine serum albumin had no detectable influence on the calcium mobilization potency. With an IC₅₀ of 210+/−30 NM (mean+/−S.E.M.; n=7), the functional antagonist ruthenium red was similar in potency to capsazepine in ability to prevent calcium mobilization by capsaicin.

A further discussion of these results can be found in Szallasi et al., 1999.

Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Various modifications of the described modes for carrying out the invention that are obvious to those skilled in the relevant arts are within the scope of the invention. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the present invention, which will be limited only by the appended claims.

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1. An assay for determining whether a compound binds capsaicin receptor, comprising the steps of: (a) contacting a human capsaicin receptor having the amino acid sequence recited in SEQ ID NO:4 with the compound and a labeled capsaicin agonist under conditions that, but for the presence of the compound, permit binding of the agonist to the receptor; (b) removing unbound labeled capsaicin agonist; (c) detecting an amount of label bound to the receptor; and (d) comparing the amount of label with an amount of label bound to the receptor in the absence of the compound, and therefrom determining whether the compound binds cansaicin recentor, wherein the receptor is present in host cells transfected with a nucleic acid vector that comprises a nucleic acid sequence encoding the amino acid sequence of SEQ ID NO:4.
 2. An assay according to claim 1, wherein the transfected host cells exhibit capsaicin agonist binding activity that is significantly greater to the p≦0.05 level than agonist activity exhibited in untransfected host cells.
 3. An assay according to claim 1, wherein the transfected host cells express at least 50,000 capsaicin receptor ligand binding sites per cell.
 4. An assay according to claim 1, wherein the capsaicin agonist is radioactively labeled.
 5. An assay according to claim 1, wherein the transfected host cells are stably transfected cells.
 6. An assay according to claim 5, wherein the transfected host cells are inducible stably transfected cells.
 7. An assay for determining whether a compound is a capsaicin receptor antagonist, comprising the steps of: (a) contacting recombinant cells with the compound and an indicator of intracellular Ca⁺² concentration, wherein the recombinant cells are transfected with a nucleic acid vector that comprises a nucleic acid sequence encoding the amino acid sequence of SEQ ID NO:4; (b) removing excess indicator to yield washed indicator-loaded cells; (c) contacting the washed indicator-loaded cells with a capsaicin receptor agonist; (d) detecting an amount of indicator in the agonist-contacted cells; and (e) comparing the amount of indicator detected with an amount of indicator detected in control cells that are contacted with indicator and agonist in the absence of the compound, and therefrom determining whether the compound is a capsaicin receptor antagonist.
 8. An assay according to claim 7, wherein the transfected cells exhibit capsaicin agonist binding activity that is significantly greater to the p≦0.05 level than agonist activity exhibited in untransfected cells.
 9. An assay according to claim 7, wherein the indicator of intracellular Ca⁺² concentration is a membrane permeable calcium sensitive dye that produces a fluorescent signal when bound to calcium.
 10. An assay according to claim 7, wherein the transfected host cells express at least 50,000 capsaicin receptor ligand binding sites per cell.
 11. An assay according to claim 7, wherein the transfected host cells are stably transfected cells.
 12. An assay according to claim 11, wherein the transfected host cells are inducible stably transfected cells.
 13. An assay according to claim 11, wherein the transfected host cells exhibit capsaicin agonist binding activity that is significantly greater to the p≦0.05 level than agonist activity exhibited in untransfected host cells.
 14. An assay according to claim 11, wherein the transfected host cells express at least 50,000 capsaicin receptor ligand binding sites per cell.
 15. An assay according to claim 11, wherein the transfected host cells are inducible stably transfected cells.
 16. An assay for determining whether a compound is a capsaicin receptor agonist, comprising the steps of: (a) contacting recombinant cells with an indicator of intracellular Ca⁺² concentration, wherein the recombinant cells are transfected with a nucleic acid vector that comprises a nucleic acid sequence encoding the amino acid sequence of SEQ ID NO:4; (b) removing excess indicator to yield washed indicator-loaded cells; (c) contacting the washed indicator-loaded cells with the compound; (d) detecting an amount of indicator in the contacted cells; and (e) comparing the amount of indicator detected with an amount of indicator detected in control cells that are contacted with indicator in the absence of the compound, and therefrom determining whether the compound is a capsaicin receptor agonist.
 17. An assay according to claim 16, wherein the transfected cells exhibit capsaicin agonist binding activity that is significantly greater to the p≦0.05 level than agonist activity exhibited in untransfected cells.
 18. An assay for determining whether a compound binds capsaicin receptor, comprising the steps of: (a) isolating a membrane preparation comprising a human capsaicin receptor having the amino acid sequence recited in SEQ ID NO:4 from host cells transfected with a nucleic acid vector that comprises a nucleic acid sequence encoding the amino acid sequence of SEQ ID NO:4; (b) contacting the human capsaicin receptor comprised by the isolated membrane preparation with the compound and a labeled capsaicin agonist under conditions that, but for the presence of the compound, permit binding of the agonist to the receptor; (c) removing unbound labeled capsaicin agonist; (d) detecting an amount of label bound to the receptor; and (e) comparing the amount of label with an amount of label bound to the receptor in the absence of the compound, and therefrom determining whether the compound binds capsaicin receptor. 